RESUMO
Increasing the fill factor (FF) and the open-circuit voltage (VOC ) simultaneously together with non-decreased short-circuit current density (JSC ) are a challenge for highly efficient Cu2 ZnSn(S,Se)4 (CZTSSe) solar cells. Aimed at such target in CZTSSe solar cells, a synergistic strategy to tailor the recombination in the bulk and at the heterojunction interface has been developed, consisting of atomic-layer deposited aluminum oxide (ALD-Al2 O3 ) and (NH4 )2 S treatment. With this strategy, deep-level CuZn defects are converted into shallower VCu defects and improved crystallinity, while the surface of the absorber is optimized by removing Zn- and Sn-related impurities and incorporating S. Consequently, the defects responsible for recombination in the bulk and at the heterojunction interface are effectively passivated, thereby prolonging the minority carrier lifetime and increasing the depletion region width, which promote carrier collection and reduce charge loss. As a consequence, the VOC deficit decreases from 0.607 to 0.547 V, and the average FF increases from 64.2% to 69.7%, especially, JSC does not decrease. Thus, the CZTSSe solar cell with the remarkable efficiency of 13.0% is fabricated. This study highlights the increased FF together with VOC simultaneously to promote the efficiency of CZTSSe solar cells, which could also be applied to other photoelectronic devices.
RESUMO
Solution method provides a low-cost and environmentally friendly route for the fabrication of Cu2 ZnSn(S,Se)4 (CZTSSe) thin-film solar cells. However, uncontrollable quality of the CZTSSe absorber layer will severely limit the device's performance. In this study, it is find that the thickness and the quality of the formed precursor is not stable because of the variation of the viscosity of the precursor solution. Combined by different characterization methods, the results disclose that such change is strongly related to the reflected color of the first coating layer during precursor growth. Further studies disclose that only by maintaining the appropriate reflected color can a well-crystallized CZTSSe film be prepared, thereby obtaining good solar cell efficiency. This semi-empirical pattern is confirmed by thin-film interference theory. Under the guidance of this method, CZTSSe absorbers with high quality are obtained easily, and the highly efficient CZTSSe solar cell can be fabricated easily. This study provides a feasible and effective strategy to obtain the optimal structure and composition of CZTSSe film toward the production of highly efficient kesterite solar cells, which can also be widely applied to the preparation of other films by solution-based method.
RESUMO
As a low-cost substitute that uses no expensive rare-earth elements for the high-efficiency Cu(In,Ga)(S,Se)2 solar cell, the Cu2 ZnSn(S,Se)4 (CZTSSe) solar cell has borrowed optimization strategies used for its predecessor to improve its device performance, including a profiled band gap and surface inversion. Indeed, there have been few reports of constructing CZTSSe absorber layers with surface inversion to improve efficiency. Here, a strategy that designs the CZTSSe absorber to attain surface modification by using n-type Ag2 ZnSnS4 is demonstrated. It has been discovered that Ag plays two major roles in the kesterite thin film devices: surface inversion and front gradient distribution. It has not only an excellent carrier transport effect and reduced probability of electron-hole recombination but also results in increased carrier separation by increasing the width of the depletion region, leading to much improved VOC and JSC . Finally, a champion CZTSSe solar cell renders efficiency as high as 12.55%, one of the highest for its type, with the open-circuit voltage deficit reduced to as low as 0.306 V (63.2% Shockley-Queisser limit). The band engineering for surface modification of the absorber and high efficiency achieved here shine a new light on the future of the CZTSSe solar cell.